1. Field of the Invention
This invention relates to additives for the proton conducting polymer electrolyte used in membranes, catalyst layers, and the like in fuel cells. In particular, it relates to additives for improved durability and performance thereof.
2. Description of the Related Art
Proton exchange membrane fuel cells (PEMFCs) convert reactants, namely fuel (such as hydrogen) and oxidant (such as oxygen or air), to generate electric power. PEMFCs generally employ a proton conducting polymer membrane electrolyte between two electrodes, namely a cathode and an anode. A structure comprising a proton conducting polymer membrane sandwiched between two electrodes is known as a membrane electrode assembly (MEA). MEA durability is one of the most important issues for the development of fuel cell systems in either stationary or transportation applications. For automotive applications, an MEA is required to demonstrate durability of about 6,000 hours.
The membrane serves as a separator to prevent mixing of reactant gases and as an electrolyte for transporting protons from anode to cathode. Perfluorosulfonic acid (PFSA) ionomer, e.g., Nafion®, has been the material of choice and the technology standard for membranes. Nafion® consists of a perfluorinated backbone that bears pendent vinyl ether side chains, terminating with SO3H.
Failure of the membrane as an electrolyte will result in decreased performance due to increased ionic resistance, and failure of the membrane as a separator will result in fuel cell failure due to mixing of anode and cathode reactant gases. The chemical degradation of PFSA membrane during fuel cell operation is proposed to proceed via the attack of hydroxyl (—OH) or peroxyl (—OOH) radical species on weak groups (such as a carboxylic acid group) on the ionomer molecular chain. The free radicals may be generated by the decomposition of hydrogen peroxide with impurities (such as Fe2+) in a Fenton type reaction. In fuel cells, hydrogen peroxide can be formed either at Pt supported on carbon black in the catalyst layers or during the oxygen reduction reaction.
The hydroxyl radical attacks the polymer unstable end groups to cause chain zipping and/or could also attack an SO3− group under dry conditions to cause polymer chain scission. Both attacks degrade the membrane and eventually lead to membrane cracking, thinning or forming of pinholes. The membrane degradation rate is accelerated significantly with increasing of the operation temperature and with decreasing inlet gas relative humidity (RH).
Different additives to the membrane electrolyte have been studied for purposes of improving the performance and/or durability of the membrane. These additives include: 1) hygroscopic particles made of metal oxide, such as silica or zirconium dioxide, heteropoly acids, phosphonate silica, etc. to improve MEA performance under low RH conditions by increasing water retention (e.g. US20070154764); 2) Pt catalyst particles dispersed in the electrolyte membrane to improve membrane durability as well as membrane performance under low RH (e.g. US20070072036); 3) metal elements or compositions containing metal elements or metal alloys that act as a free radical scavenger or hydrogen peroxide decomposition catalyst (e.g. US2004043283); 4) phenol type antioxidants where the antioxidant can be a small molecule or a polymer (e.g. US2006046120); 5) organic crown compounds (e.g. US20060222921) or macrocyclic compounds containing metal or metalloids (e.g. WO2007144633); and 6) cation chelating agents to reduce formation of free radicals (e.g. U.S. 6,607,856).
Additives are also disclosed in WO2005060039 to address the problem in PEM fuel cell durability of premature failure of the ion-exchange membrane. The degradation of the ion-exchange membrane by reactive hydrogen peroxide species can be reduced or eliminated by the presence of an additive in the anode, cathode or ion-exchange membrane. The additive may be a radical scavenger, a membrane cross-linker, a hydrogen peroxide decomposition catalyst and/or a hydrogen peroxide stabilizer. The presence of the additive in the membrane electrode assembly (MEA) may however result in reduced performance of the PEM fuel cell. In particular, suggested additives include an organometallic Mn(II) or Mn(III) complex having an organic ligand selected from CyDTA, ENTMP, gluconate, N,N′-bis(salicylidene)propylenediamine, porphyrins, phthalocyanines, phenanthroline, hydrazine, pyrocatechol-3,5-disulphonic acid disodium salt, triethylenetetraamine, shiff base macrocycles and EDDA.
In commonly owned U.S. patent application Ser. No. 12/615,671, with the title “Composite Proton Conducting Membrane with Low Degradation and Membrane Electrode Assembly for Fuel Cells” and filed on Nov. 10, 2009, certain ligand additives (e.g. 1,10-phenanthroline or 2,2′-bipyridine) were disclosed that meet many of these needs. The use of these ligand additives in the membrane and/or catalyst layers can improve durability but, depending on testing conditions, there may be a modest penalty in fuel cell performance (e.g. 3 times better stability might be obtained but with a 20 mV loss in voltage under load). Preferably, both durability and performance of fuel cells would be improved with appropriate additives.
Accordingly, there remains a need for improved additive technology that provides additional resistance of MEAs, and specifically PFSA membranes of the MEAs, to degradation, resulting in improved MEA durability and performance under low RH in a fuel cell. This invention fulfills these needs and provides further related advantages.